A semiconductor laser included in an optical pickup used in reproducing/recording an optical disk attains a higher recording density in the optical disk as the lasing wavelength is shorter. Therefore, a nitride semiconductor laser lasing at a wavelength ranging from a blue to violet region has been developed, and an optical pickup using such a nitride semiconductor laser has been put to practical use. Also, application of a nitride semiconductor laser lasing at a wavelength of the UV region to a solid-state lighting device in which a fluorescent material is excited with UV has been examined, and such a solid-state lighting device is expected to take the place of a fluorescent lamp.
It is known that catastrophic optical damage (COD) is caused on a resonator end face in such a nitride semiconductor laser and thus prevents the laser from attaining high output in the same manner as in a conventional AlGaAs-based infrared semiconductor laser or a conventional AlGaInP-based red semiconductor laser. The COD is phenomenon that a resonator end face is damaged through a positive feedback function as follows: Non-radiative recombination of electron-hole pairs is caused in a larger amount in the vicinity of the end face where interface state density is higher than in the inside of the resonator, the temperature is increased by the non-radiative recombination so as to reduce the band gap and cause light absorption, and the light absorption further increases the temperature.
In the conventional infrared or red semiconductor laser, in order to prevent the occurrence of the COD, an impurity is diffused in a portion of a multiple quantum well active layer disposed in the vicinity of a resonator end face for disordering a well layer and a barrier layer, and thus, a window structure in which the band gap of the active layer is larger merely in the vicinity of the end face is employed. In a nitride semiconductor, however, it is difficult to disorder a multiple quantum well, and hence, the window structure is difficult to obtain in the aforementioned manner. As an alternative method for forming the window structure, a method in which a nitride semiconductor layer with a large band gap is previously buried through crystal growth in a portion corresponding to an end face so as to obtain an end face window structure by cleaving this portion is disclosed (see Patent Document 1).
Alternatively, instead of forming the window structure, the COD may be suppressed by preventing a current from being injected into a portion in the vicinity of an end face. With respect to a nitride semiconductor laser, a technique in which an impurity is introduced into a portion in the vicinity of a resonator end face for increasing the resistance so as to prevent a current from passing the portion (see Patent Document 2) and a technique in which a Schottky electrode is formed instead of an ohmic electrode in the vicinity of a resonator end face for preventing injection of a current (see Patent Document 3) are disclosed. Now, the technique disclosed in Patent Document 2 will be simply described with reference to a drawing.
In the fabrication of a semiconductor laser having an ultimate structure shown in FIG. 10, an n-type contact layer 1002 of GaN, an n-type cladding layer 1003 of AlGaN, a multiple quantum well active layer 1004, a p-type cladding layer 1005 of AlGaN and a p-type contact layer 1006 of GaN are successively formed on a sapphire substrate 1001. Si is used as an n-type dopant for the n-type contact layer 1002 and the n-type cladding layer 1003, and Mg is used as a p-type dopant for the p-type cladding layer 1005 and the p-type contact layer 1006. Next, a ZnO film is formed on the p-type contact layer 1006 merely in the vicinity of a resonator end face, and annealing is performed with a thermal diffusion furnace. Thus, Zn included in the ZnO film is thermally diffused downward so as to be introduced through the p-type contact layer 1006 into the p-type cladding layer 1005, the active layer 1004 and even a part of the n-type cladding layer 1003. As a result, a high-resistance region 1007 is formed in the vicinity of the resonator end face.
After removing the ZnO film, a ridge is formed from a projected portion of the p-type cladding layer 1005 and the p-type contact layer 1006, and a SiO2 current blocking layer 1008 having an opening on a portion of the p-type contact layer 1006 excluding the high-resistance region 1007 and on a part of the n-type contact layer 1002 is formed. Thereafter, a p-side pad electrode 1009 is formed on a p-side ohmic electrode (not shown), and an n-side pad electrode 1011 is formed on an n-side ohmic electrode 1010, resulting in completing the semiconductor laser shown in FIG. 10. This semiconductor laser has an advantage that the COD is minimally caused because no current is injected into the high-resistance region 1007 formed in the vicinity of the resonator end face.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-60298
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-305353
Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-31894
Patent Document 4: Japanese Laid-Open Patent Publication No. 2005-217415